Censuses were taken in the colonies prior to the Constitution's ratification but the first official U.S. population census after the American Revolution was taken in 1790, and every ten years since then. Its official name is the Decennial Census of Population and Housing, and the most recent one was conducted in 2020. For years between the decennial censuses, the American Community Survey (ACS) is an ongoing survey that provides information on a yearly basis. The parent agency of the Census Bureau is the U.S. Department of Commerce.
The Decennial Census (which is usually the one meant by "the Census" in popular language) aims to count the entire U.S, population and is conducted once every 10 years in years ending in "0", as mandated by Article I, Section 2 of the Constitution. The data collected determines the number of seats each state has in the U.S. House of Representatives and affects allocation of federal funds to states, so it has considerable political and economic impact. The first decennial census in 1790 was a "simple" count of 3.9 million people. The 2010 census was the first census that recorded a population exceeding 300 million (see also the population clock for the latest known count). Future censuses may move away from the "count everyone" approach to more utilization of other, existing administrative records to reduce costs.
Summary statistics or aggregated data from the collected "raw" data have been made available by the U.S. Census Bureau in various forms over time, beginning with tables in print publications, moving on to computer tapes and discs, and then different Internet-based platforms, such as DataFerrett (decommissioned on June 30, 2020). The latest incarnation is data.census.gov. But there are other, for-fee platforms providing access, sometimes arguably more user-friendly, to Census data - for American University users, see subscription based tools.
The American Community Survey (ACS) was begun in 2005 and is conducted every year. It polls a representative, randomized sample of about three million American households about demographics, habits, languages spoken, occupation, housing and various other categories. The Census Bureau selects a random sample of addresses to be included in the ACS. Data are collected by internet, mail, telephone interviews and in-person interviews. The resulting numbers are released without identifying individuals, and offer detailed current demographic data. ACS data is also available through data.census.gov.
For an overview of the differences between both, see: The Importance of the American Community Survey and the Decennial Census.
In addition to the two aforementioned population-and-housing-focused programs, the Census Bureau also administers a Census of Governments at the U.S. sub-national levels and an Economic Census.
Furthermore, the Census Bureau conducts some surveys on behalf of other federal agencies; for example, the American Housing Survey (AHS), sponsored by the Department of Housing and Urban Development (HUD), or the monthly Current Population Survey (CPS) for the Bureau of Labor Statistics (BLS). Collaborations between Census and other agencies also results in statistics such as the U.S. monthly international trade deficit and in the North American Industry Classification System (NAICS).
Lastly, Census data also feeds into other agencies' own activities; for example, the Provisional Mortality Data — United States, 2020 by the Centers for Disease Control and Prevention (CDC).
While the various publicly available and subscription-based online tools make summary statistics or aggregated data available, and even de-identified Public Use Microdata with certain limitations, access to restricted-use microdata from the Census Bureau and several other U.S. statistical agencies requires application to work with such data at one of the Federal Statistical Research Data Centers (FSRDCs). This application process is generally open only to faculty and doctoral students or researchers with an equivalent status, and typically takes at least several months and proper justification. Two FSRDCs (of thirty around the U.S.) in Washington DC, as of July 2021, are at the Federal Reserve Board and at Georgetown University.
